In recording a video signal on an optical disc or in a video tape recorder in the form of an analog signal and reproducing the same, a time base corrector is used for removing time base fluctuation of the reproduced video signal.
FIG. 1 is a schematic block diagram showing one example of a conventional time base corrector. Referring to FIG. 1, a terminal 26 receives a reproduced video signal (still picture signal) having a time base fluctuated. This reproduced video signal is applied to an A/D convertor 21 and a writing clock generating circuit 24. Writing clock generating circuit 24 separates a horizontal synchronizing signal from the reproduced video signal and also generates a writing clock signal W.multidot.CK corresponding to a time base fluctuation of the reproduced video signal. In response to the writing clock signal W.multidot.CK, A/D converter 21 samples the reproduced video signal and digitalizes the same to be applied to a memory 22. Memory 22 writes the digitalized video signal in response to the writing clock signal W.multidot.CK having the same time base fluctuation.
Meanwhile, an external reference synchronizing signal is applied to a reading clock generating circuit 25, which circuit generates a reading clock signal R.multidot.CK synchronized with the external reference synchronizing signal having a fixed time base, which clock signal is applied to memory 22 and a D/A converter 23. In response to the reading clock signal R.multidot.CK, memory 22 reads the stored digital video signal and applies the same to D/A converter 23. In response to this reading clock signal R.multidot.CK, D/A converter 23 converts the digital video signal to an analog signal. Therefore, the video signal output from D/A converter 23 to an output terminal 27 has a fixed time base.
Time base correcting capability of such a time base corrector as shown in FIG. 1 depends on whether the writing clock signal W.multidot.CK can be generated or not which precisely corresponds to a time base fluctuation of the reproduced video signal. A conventionally proposed common BCO (burst controlled oscillator) using such elements as a crystal oscillator, a coil and a capacitor is not satisfactory and therefore a circuit is required having a wide frequency response range and a high response speed. Such a BCO is essentially liable to become unstable, affected by noise, waveform distortion, drop out or skew.
Thus, various improvements have been made in a burst gate circuit or a synchronization separating circuit included in a time base corrector in order to prevent attenuation of a time base fluctuation component of an input video signal while reducing the effect of noise or the like. For example, used as a horizontal synchronizing signal separating circuit are a delay circuit, a synchronization gate circuit employing a gate signal and a flywheel oscillator, and such a circuit as detecting drop out to mute the drop out noise in a video signal and inhibiting synchronization separating or clamping. A wide-band circuit is used as a color burst gate circuit for amplifying and separating a synchronizing signal and a color burst signal.
FIG. 2 shows one example of a case where a precise reading clock signal R.multidot.CK is generated by using the separated synchronizing signal and color burst signal.
In a writing clock generating circuit 24 shown in FIG. 2, a horizontal synchronizing signal is applied to a phase comparator 31 through a terminal 40. Phase comparator 31 compares the phase of the horizontal synchronizing signal with that of a signal obtained by frequency-dividing an output of a variable voltage controlled oscillator (VCO) 33. A frequency-dividing circuit 34 applies an error voltage to VCO 33 through a loop filter 32. Phase comparator 31, loop filter 32, VCO 33 and frequency-dividing circuit 34 constitute a PLL loop. Loop filter 32 is provided in order to prevent oscillation of the PLL loop, by which loop filter 32, even at a sudden phase fluctuation of the video signal, the output of VCO 33 scarcely follows the phase fluctuation, resulting in a phase being independent of a sub carrier.
The output of VCO 33 is applied to a frequency-dividing circuit 35 wherein the frequency of the output is divided by four to output a 3f.sub.SC (f.sub.SC is a sub carrier frequency) signal. Frequency-dividing circuit 35 is reset in response to one pulse of a color burst signal inputted to a terminal 41. Through the resetting operation, a phase difference between the 3f.sub.SC signal, the frequency-divided output of frequency-dividing circuit 35, and the color burst signal becomes 30.degree. or less in color sub carrier phase. When using the 1/4 frequency-divided output of frequency-dividing circuit 35 is applied to a phase modulator 36 wherein the phase of the output is modulated, which modulated output is applied to a frequency-dividing circuit 37 wherein the frequency thereof is divided by 3 to attain the same cycle of the color sub carrier. The 1/3 frequency-divided output of frequency-dividing circuit 37 is applied to a phase comparator 38. Phase comparator 38, with the color burst signal applied thereto, compares phases of the 1/3 frequency-divided output signal and the color burst signal to apply an error voltage to phase modulator 36 through a loop filter 39. In response to the error voltage, phase modulator 36 modulates the phase of the 3f.sub.SC signal to output to a terminal 42 a writing clock signal W.multidot.CK following the input video signal and having a phase locked to the horizontal synchronizing signal.
Since in time base corrector 20 using writing clock generating circuit 24 shown in FIG. 2, however, feed back control is employed wherein loop filters 32 and 39 are used in VCO 33 for generating a signal from which a writing clock signal is generated and in phase modulating circuit 36 for phase-locking the output of frequency-dividing circuit 35 to a color burst signal, respectively, even such improvements as described above do not allow a writing clock generating circuit having an high response speed to be achieved. In the case of an optical disc still picture file, in particular, it is impossible to completely correcting a time base at a high speed through feed back control because one frame of still picture is reproduced only for a short time period and a rotation jitter of the optical disc has a high frequency component.